1. Field of the Invention
The invention relates generally to a method for determining the position of a medical instrument partially introduced into an object to be examined, in a three-dimensional image data set of the object to be examined, and also relates to a device for carrying out such a method.
2. Description of Related Art
A method and device for determining the position of a medical instrument in a two-dimensional image data set are known from EP 857 461 A2. Therein, X-ray images of the examination zone of an object to be examined, for example a patient, are acquired intra-operatively by means of a C-arm X-ray device, while the position of the object to be examined, or the patient table, and the medical instrument relative to the X-ray device is measured at the same time by means of an optical position measuring device. The position of the medical instrument can subsequently be transformed into a position relative to one or more of the acquired X-ray images, so that the respective instantaneous position can always be reproduced in one or more X-ray images. A method of this kind can serve as a navigational tool for the physician during the treatment of a patient. However, it has the drawback that no intra-operative three-dimensional image information is available for the navigation. However, an intervention can be planned on the basis of a pre-operative three-dimensional data set, but only two-dimensional X-ray images can be acquired intra-operatively and the position of the medical instrument can be determined and indicated only in such intra-operatively acquired two-dimensional X-ray images.
Furthermore, methods are known in which the position of a medical instrument is intra-operatively determined so as to be transformed into a position relative to a pre-operatively acquired three-dimensional image data set. However, during the acquisition of the three-dimensional image data set the patient must be provided pre-operatively with special markers which are also reproduced in the three-dimensional image data set and are approached by a special pointer directly before the operation so as to determine their positions in space. An intra-operatively measured spatial position of a medical instrument can then be transformed into a position relative to the three-dimensional image data set by utilizing the positions of such markers which are thus known in spatial co-ordinates and in 3D image coordinates. Such methods, however, have the drawback that no instantaneous image information concerning the anatomy of the patient is used whereas during the intervention the anatomy regularly changes relative to the state of the anatomy during the pre-operative acquisition of the 3D image data set.
Therefore, it is an object of the present invention to provide a method for determining the position of a medical instrument, partially introduced into an object to be examined, in a three-dimensional image data set of the object to be examined that avoids the above-mentioned drawbacks and enables a high accuracy to be achieved at an expenditure which is as small as possible. Moreover, it is also an object of the present invention to provide a device which is suitable for carrying out such a method.
These objects are achieved by means of a method including the steps of acquiring a three-dimensional image data set of the object, acquiring a two-dimensional X-ray image of the object, determining spatial positions of the X-ray image and the medical instrument, determining a spatial correlation between the acquired X-ray image and the acquired three-dimensional image data set, and determining the position of the medical instrument in the three-dimensional image data set from the determined spatial position of the medical instrument based on the determined spatial correlation between the acquired X-ray image and the acquired three-dimensional image data set. These objects are also achieved by means of a device comprising an X-ray device for acquiring a two-dimensional X-ray image of the object, a position measuring device for measuring the spatial positions of the X-ray image and the medical instrument when introduced into the object, and a processor for determining the spatial correlation between the X-ray image and the three-dimensional image data set. The processor determines the position of the medical instrument in the three-dimensional image data set from the spatial position of the medical instrument by means of the spatial correlation between the X-ray image and the three-dimensional image data set.
The present invention is based on the recognition of the fact that an intra-operative two-dimensional X-ray image can be advantageously used to transform the intra-operatively measured position of the medical instrument into a position relative to a three-dimensional image data set which will usually have been pre-operatively acquired. According to the invention, to this end, not only the position of the medical instrument but the position in space of the X-ray image is measured intra-operatively. Using a suitable registration method, the spatial correlation between this X-ray image and the three-dimensional image data set is subsequently determined, thus yielding something resembling the spatial position of the three-dimensional image data set. This knowledge enables a simple determination of the position of the medical instrument relative to the three-dimensional image data set because the spatial position of the medical instrument has been acquired directly before that. The present invention thus enables a simple determination of the position of a medical instrument in a three-dimensional image data set without it being necessary to provide the patient during the acquisition of the image data set with special markers that are to be reproduced and must be registered again immediately before the operation. Moreover, intra-operatively acquired image data providing exact information concerning the anatomy of the patient are processed, so that the accuracy of the determination of the position is enhanced.
Another embodiment of the present invention includes means for determining the spatial position of the X-ray image; such means can also be used to determine the spatial position of the medical instrument. The position measuring device used for this purpose may be of a variety of constructions; for example, it may include optical cameras, infrared cameras and/or electromagnetic detectors that are capable of determining the three-dimensional position of corresponding markers, for example, optical light-emitting diodes, infrared diodes or electromagnetic transmitters.
Additional embodiments provide for further possibilities for determining the spatial correlation between the X-ray image and the three-dimensional image data set. To this end, the overall three-dimensional image data set, or one or more sub-volumes or individual objects or structures that are particularly prominent in the image data set or individual voxels of the image data set are compared with the X-ray image or searched in the X-ray image. This operation is preferably performed iteratively. Such an advantageous method of comparison is known from EP 880 109 A2 to which reference is made explicitly herein and whose disclosure is considered to be included in the present application. Pseudo-projection images are thus formed from the three-dimensional image data set and compared with the X-ray image, the parameters underlying the formation of the pseudo-projection image, for example the imaging scale, projection direction etc., being iteratively varied until the pseudo-projection image and the X-ray image match as well as possible. The spatial correlation between the X-ray image and the 3D image data set is thus found. can be executed intra-operatively and continuously, and hence it can serve as a navigational tool for the physician during the treatment of a patient and can continuously deliver instantaneous information concerning the anatomy and the position of the medical instrument.
The three-dimensional image data set may be used to derive an image in which the position of the medical instrument or the instrument itself is reproduced. This also serves as to aid the attending physician during an operation. Different images can then be formed, for example, layer images or projection images that were formed from the three-dimensional image data set and cannot be formed by means of the intra-operatively used X-ray device, for example, combination images from pre-operatively acquired and intra-operatively acquired image data, vascular systems or pre-operatively determined navigation plans.
The three-dimensional image data set may be acquired pre-operatively by means of an arbitrary imaging device and an X-ray fluoroscopy device, for example a C-arm X-ray device, is used intra-operatively. As a result, images from different imaging modalities and with a different information contents can thus also be intra-operatively presented to the physician during the treatment.
A device according to the invention which is suitable for carrying out the described method particularly advantageously includes an X-ray device, a position measuring device and an arithmetic unit or processing unit.